1. Warm Up
An example of a geologic event that could be recorded in rocks is ____.
a. a lunar eclipse c. a civil war
b. a volcanic eruption d. the occurrence of sun spots
Nicolaus Steno proposed the most basic principle of relative dating, the law of ____.
a. gravity c. superposition
b. fossil succession d. correlation
If a layer of sandstone is in contact with a mass of granite that contains small pieces of the sandstone, which rock is older?
a. Both are the same age.
b. the granite
c. the sandstone
d. It is not possible to determine which rock is older from this information.
Answers: 1) b. 2) c. 3) c.

2. Fossils: Evidence of Past Life
Chapter 12, Section 2

3. Fossil Formation
Fossil – the remains or traces of an organism preserved from the geologic past
The type of fossil that is formed is determined by the conditions under which an organism died and how it was buried
Some remains of animals—such as teeth, bones, and shells—may not have been altered hardly at all over time (unaltered remains)
The remains of an organism are likely to be changed over time (altered remains)
Fossils often become petrified, mineral-rich water soaks into small cavities and pores of the original organism, and replace the original material

4. Molds and casts are another common type of fossil; it is created when a shell or other structure is buried under sediment and then dissolved by groundwater
Carbonization is effective in preserving leaves and delicate mineral forms, occurs when an organism is buried under fine sediment and subjected to pressures which squeeze out its liquid and gases
Trace fossils are indirect evidence of prehistoric life (tracks, burrows, dung, etc.)
Two conditions are important for preservation: rapid burial and the possession of hard parts

5. Fossil Formation

6. Fossil Types

7. Concept Check What are three types of fossils?
Petrified remains, molds, casts, impressions, and trace fossils

8. Fossils and Correlation
In the late 18th Century, William Smith found that fossils were not randomly distributed throughout the rock layers, but each layer contained a distinct assortment of fossils that did not occur in other layers
The principle of fossil succession states that fossil organisms succeed one another in a definite and determinable order; any time period can be recognized by its fossil content
Based on the rock record from around the world, geologists have identified an order of fossils: an Age of Trilobites, an Age of Fishes, an Age of Coal Swamps, an Age of Reptiles, and an Age of Mammals

9. Index Fossil – a fossil that is associated with a particular span of geologic time
Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers
Their presence provides an important method of matching rocks of the same age
Fossils can also be used to interpret and describe ancient environments

10. Fossils and Correlation

11. Index Fossils

12. Assignment Read Chapter 12, Section 2 (pg. 343-346)
Do Chapter 12 Assessment #1-33 (pg )
For Section 2: Do #’s 3, 6, 16-18, 25, 27, 28, 30, & 31